Several anti-monarchy protesters were arrested in central London before the coronation of King Charles III, their group has said, as the Metropolitan Police force faces scrutiny for its approach toward demonstrators at the historic event.

Thousands gathered in central London on Saturday to celebrate the once-in-a-generation event. But it also drew demonstrators, with protesters wearing yellow T-shirts booing and shouting “Not My King” throughout the morning.

Republic, Britain’s largest anti-monarchy group complained that police — without providing any reason — arrested the organizers of Saturday’s anti-monarchy protest.